docs: Document that adding new optimizer rules are expensive

datafusion/optimizer/src/optimizer.rs

@@ -236,6 +236,15 @@ impl Default for Optimizer {
 impl Optimizer {
     /// Create a new optimizer using the recommended list of rules
     pub fn new() -> Self {
+        // NOTEs:
+        // - The order of rules in this list is important, as it determines the
+        //   order in which they are applied.
+        // - Adding a new rule here is expensive as it will be applied to all
+        //   queries, and will likely increase the optimization time. Please extend
+        //   existing rules when possible, rather than adding a new rule.
+        //   If you do add a new rule considering having aggressive no-op paths
+        //   (e.g. if the plan doesn't contain any of the nodes you are looking for
+        //    return `Transformed::no`; only works if you control the traversal).
         let rules: Vec<Arc<dyn OptimizerRule + Sync + Send>> = vec![
             Arc::new(RewriteSetComparison::new()),
             Arc::new(OptimizeUnions::new()),

datafusion/physical-optimizer/src/optimizer.rs

@@ -82,6 +82,12 @@ impl Default for PhysicalOptimizer {
 impl PhysicalOptimizer {
     /// Create a new optimizer using the recommended list of rules
     pub fn new() -> Self {
+        // NOTEs:
+        // - The order of rules in this list is important, as it determines the
+        //   order in which they are applied.
+        // - Adding a new rule here is expensive as it will be applied to all
+        //   queries, and will likely increase the optimization time. Please extend
+        //   existing rules when possible, rather than adding a new rule.
         let rules: Vec<Arc<dyn PhysicalOptimizerRule + Send + Sync>> = vec![
             // If there is a output requirement of the query, make sure that
             // this information is not lost across different rules during optimization.